Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
  • B23K26/03—Observing, e.g. monitoring, the workpiece
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
  • B23K26/04—Automatically aligning, aiming or focusing the laser beam, e.g. using the back-scattered light
  • B23K26/042—Automatically aligning the laser beam
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
  • B23K26/03—Observing, e.g. monitoring, the workpiece
  • B23K26/032—Observing, e.g. monitoring, the workpiece using optical means
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/0006—Working by laser beam, e.g. welding, cutting or boring taking account of the properties of the material involved
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
  • B23K26/04—Automatically aligning, aiming or focusing the laser beam, e.g. using the back-scattered light
  • B23K26/046—Automatically focusing the laser beam
  • B23K26/048—Automatically focusing the laser beam by controlling the distance between laser head and workpiece
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
  • B23K26/06—Shaping the laser beam, e.g. by masks or multi-focusing
  • B23K26/062—Shaping the laser beam, e.g. by masks or multi-focusing by direct control of the laser beam
  • B23K26/0622—Shaping the laser beam, e.g. by masks or multi-focusing by direct control of the laser beam by shaping pulses
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
  • B23K26/06—Shaping the laser beam, e.g. by masks or multi-focusing
  • B23K26/0665—Shaping the laser beam, e.g. by masks or multi-focusing by beam condensation on the workpiece, e.g. for focusing
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/20—Bonding
  • B23K26/21—Bonding by welding
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/352—Working by laser beam, e.g. welding, cutting or boring for surface treatment
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/36—Removing material
  • B23K26/38—Removing material by boring or cutting
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K2103/00—Materials to be soldered, welded or cut
  • B23K2103/50—Inorganic material, e.g. metals, not provided for in B23K2103/02 – B23K2103/26

Definitions

• This invention relates to optimising the focus of a fibre laser.
• a high brightness fibre laser that is, a fibre laser of brightness greater than 5 about M 2 ⁇ 10
• a typical processing arrangement has a lens with variable focus position, a gas assist delivered via a nozzle and the workpiece itself usually mounted on 10 some form of controllable translation stage.
• a high powered solid state laser of 200W to 4kW .
• Lasers of this type would be relatively low brightness M >10 and be delivered in a large mode area multimode fibre, eg 400 ⁇ 0.1 measured Numerical Aperture (NA) equivalent to an M of 60.
• NA Numerical Aperture
• the working spot size is at least an order of magnitude larger than that of a fibre laser but the confocal parameter is about 20 times larger consequently focus is less critical.
• the parameter M is defined further later on in this document.
• Single mode fibre lasers are now capable of up to 10KW in a single mode.
• the beam quality from a single mode fibre laser is a near perfect M of 1 consequently the output of a fibre laser can be focussed to a much smaller spot than conventional solid state lasers.
• This decreased spot size typically as small as 10 ⁇ diameter, creates similar 25 intensities as conventional lasers but at a much lower power.
• a beam with an M of 1 and diameter 10 ⁇ will have a confocal parameter of +/- 75 ⁇ illustrating the potential tolerance requirements of processing with a beam of this nature.
• the present invention arose in an attempt to provide an improved method of optimising the focus of a high brightness fibre laser.
• a method of optimising the focus of a fibre laser comprising positioning the output of a fibre laser relative to a workpiece; measuring at least a portion of back reflected radiation from the workpiece; changing the relative position of the output and the workpiece one or more times and measuring at least a portion of back reflected radiation from the workpiece each time, and analysing the resulting measured back-reflected radiations to determine optimum focus.
• the method comprises determining an integral back-reflected radiation at each relative position and using the integrals to determine optimum focus.
• the method comprises generating pulses and measuring the back-reflective radiation resulting from the pulse, and integrating each set of back- reflection results as the relative position of the workpiece and output is changed.
• the method may be done using an apparatus as described in Patent Application No WO 2009/112815.
• material processing system apparatus comprising a fibre laser having an output which focuses a laser beam from the laser onto a workpiece, the output and workpiece being moveable relative to each other; means for monitoring at least part of the back reflection; means for analysing the back-reflection at each of a selection of relative positions of the output and workpiece and means for using the analysis to determine optimum focus.
• the apparatus comprises means for integrating the back reflection and each relative position for using the integrals to determine optimum focus.
• Figure 1 shows a fibre laser system for processing a workpiece
• Figure 1(a) shows a cross-section through a delivery fibre
• Figure 2 shows part of a laser delivery system
• Figure 3 shows a light stripping means
• Figure 4(a) to 4(d) show respective plots of detected back-reflected infrared radiation from laser pulse as the relative position of a workpiece and laser output is changed;
• Figure 5 shows a method of optimising focus
• Figure 6 shows two plots of results with a lens displaced by 100 ⁇ between the plots
• Figure 7 shows examples of use of the invention with different sizes of focus spot
• Figure 8 shows an example of pulse regime for tracking a temporal change in focal position
• Figure 9 shows a back reflected signal
• Figure 10 shows an example of a relatively good optical performance
• Figure 11 shows an example of a relatively worse optical performance.
• FIG. 1 shows schematically a material processing apparatus using a fibre laser.
• a fibre laser 1 is used to generate a laser beam in well known manner
• a typical fibre laser includes means for transmitting pumping radiation (light) from one or more pumping laser diodes through the cladding of a double clad fibre, the pump light is absorbed in the doped core of the fibre, diffraction gratings are spliced to the fibre amplifier to form a resonant cavity.
• the laser beam emitted from the fibre laser is applied via a double clad single mode delivery fibre 2 to a processing unit 3 which generally includes optics for collimation and focusing of the laser beam.
• the laser beam is then focused through a lens arrangement 4 to a workpiece 5 where it is used for processing the workpiece.
• This processing may typically include cutting, welding, surface modification or other processing steps.
• the interaction of the light with the material varies depending on a wide range of parameters as discussed.
• the back- reflected radiation from the workpiece is, in embodiments of the invention, used to control the process.
• Figure 1 includes schematically, as part of the fibre laser, an integral monitor 6 of back-reflection. Back-reflection from this integral monitor is applied to a system control and analysis device 7.
• the fibre laser output is delivered via a double clad single mode delivery fibre 2 which may, for instance, have a core diameter of 10 microns and a first cladding diameter of 200 ⁇ having numerical apertures of 0.08 and 0.46 respectively.
• Back-reflection at the laser wavelength is caused by reflection from the surface of the workpiece that the laser output is focused onto. Back-reflections at other wavelengths can be generated by the plasma formed from interaction of the beam with the workpiece. A portion of this back-reflection is collected back into the imaging optics and transmitted back into the first cladding of the delivery fibre. This back-reflection propagates in the first cladding of the delivery fibre back into the laser.
• the single mode delivery fibre 2 comprises a core 8, a first cladding layer 9 and an outer cladding layer 10.
• the laser output is transmitted to the workpiece in the core 8.
• a significant proportion of the back-reflected light propagates in the first cladding 9 guided by the outer cladding 10 which has a lower refractive index.
• the reflected signal captured by the delivery fibre can be extracted from the 10/200 ⁇ fibre by stripping the fibre of its outer low index coating and then potting this area in a high index material.
• the high index material may, for example, be Norland optical adhesive having a refractive index of 1.56.
• the guided light in the multimode cladding of the delivery fibre is stripped out over a distance of a few millimetres but this may vary. The stripped light is then monitored by any convenient monitoring device. This may typically be any opto electronic device such as a photodiode.
• This outer cladding is most preferably partially stripped somewhere in an unbroken length of the delivery fibre away from any splice points: this arrangement maximises the discrimination between the forward-going light emitted by the laser and the back-reflected light in the cladding.
• Figure 2 is an example of the latter, where the splice is located in the stripped region 11 of the double clad single mode delivery fibre, where the outer cladding layer 10 is stripped away. This enables the back-reflected light which was in the first cladding layer to be detected by a suitable opto electronic device and used for monitoring the process.
• the outer cladding layer is stripped in the vicinity of a splice point, it is most preferable that discrimination is made between the forward signal arising from scatter of the splice 12 itself and the reverse cladding mode. This is because inevitably at a splice point, some of the desired laser signal travelling in the forward direction F will be scattered at the splice point and this is not the radiation that it is desired to monitor. The desired radiation will be the radiation moving in the backwards direction B.
• a cladding mode stripper (CMS) is most preferably used.
• FIG 3 shows one example of a CMS. This comprises a component 13 that is placed in the vicinity of the splice point and stripped portion of the fibre and which comprises two spatially displaced portals 14 and 15. It is positioned with respect to the stripped portion of the fibre so that the portal 14 overlies the splice point 12. The portion 15 lies a distance away from that (typically nearer the workpiece/beam delivery end). In a typical embodiment, the distance between the centres of the portal is around 13mm and the radius of each portal is around 7mm. The fibre is potted within this component.
• the portals are generally circular in the embodiment shown. They may be elliptical, or any other shape.
• the device 13 has a longitudinal groove 20 into which the optical delivery fibre, including the stripped portion, is located (potted).
• the stripped portion of the fibre has between A and B in the figure, with portal 14 overlying a splice point and portal 15 spaced apart from this.
• a light detector such as one or more photodiodes or other opto electronic means, is arranged to detect light contained by the portal 15 which is spaced from the splice.
• the portal 14 serves to contain any light which is scattered at the splice position itself (which will be mainly laser radiation and not the back-reflection which it is intended to monitor) and this is therefore isolated from the monitor point.
• a target material (herein known as a workpiece although it may of course be just a sample target to set up focus, or any other target, or a piece of material which is specifically desired to be worked on) is placed within the focus region.
• the target material is preferably of a stainless steel, although it may be of other material.
• the nozzle height relative to the workpiece can be set so that the focus found is set relative to the gas tip.
• the fibre laser is arranged to transmit fibre pulses via the processing unit 3 and lens 4 to the workpiece 6 and is spaced such that the focus lens to workpiece distance F can be varied in axial direction Z (or having a component in direction Z). This can be by moving the focus lens, and/or by moving the workpiece.
• the term Output' is used in the specification to imply the part at which laser radiation from the fibre is launched into free space before it impinges upon the workpiece.
• Figures 4(a) to 4(d) show typical values of back-reflected radiation from a pulsed laser output.
• the fibre laser is arranged to produce 100 ⁇ 8 50 W peak square pulses.
• the IR back-reflection from this pulse is measured over a period of time (140 ⁇ 8 shown in each figure) and the back-reflection measured is the infrared component.
• Figure 4(a) shows the result when the workpiece is 0.1 mm out of focus. That is, the workpiece is positioned 0.1 mm away from the focus position of the lens 4.
• Figure 4(b) shows a resulting waveform when the workpiece is 0.2 mm out of focus
• Figure 4(c) when it is 0.3 mm out of focus
• Figure 4(d) when it is 0.4 mm out of focus.
• Figure 5 shows a flow chart illustrating steps used in an exemplary method using the apparatus of Figures 1 to 3.
• the workpiece is relatively positioned a first distance X from the focus lens. This step is not specifically shown in figure.
• the laser is then begun to be operated to generate laser energy, preferably pulsed laser energy. In one particular embodiment, it generates 100 ⁇ 8 50 W peak square pulses at step 51.
• the back-reflected IR, or part of it, is captured by the CMS (cladding mode stripper), eg that of Figure 5, or other means for removing radiation.
• the captured light is measured and the back-reflected waveform is integrated by software within the fibre laser 1 at step 52.
• FIG. 6 By continually looping through a range of distances a number of values of the integral can be measured and these values are plotted at step 56. Examples of this resulting plot are shown in Figure 6.
• the plot show the integrated value of the back-reflected signal against the distance of the focus lens relative to the target.
• a parabola of this type is typical when the back-reflected signal is plotted against the focus position.
• a first plot 61 is a reference plot and that the second plot 62 shows a plot where the lens is displaced by 100 ⁇ ., illustrating how the vertex of the parabola is shifted by this focal length change .
• finding the null of the curve the focus can be found to within the confocal parameter of the beam. Finding the vertex of the parabola achieves the highest accuracy for the position of the focus with a precision much smaller than the confocal parameter of the beam
• Figure 7 shows a similar plot for different focal length lenses.
• plot 71 shows a 100 mm focal length lens and plot 62 shows a 160 mm focal length lens. Again, minima are achieved for both and thus the precise focal length can be easily established for precise positioning.
• the parameters for three beams are shown:
• the focal position can be found to within the confocal parameter of the beam itself. Note that there is a greater proportion of back- reflected light deposited in the CMS port when the lens to workpiece distance is greater than the focal length of the lens than when it is shorter. This is seen in both Figures 6 and 7 where there is asymmetry between the right and left hand sides of the focal point at 0 mm. This asymmetry can be used to give directionality to the focus finding operation.
• the choice of the probe pulse energy may be of importance in some embodiments. The energy should be chosen so as to enable boiling point to be reached on the target. This resulting phase change results in near 100% absorption of the laser light and zero reflection at optimum focus.
• the peak power of the laser is set too high than the phase change will occur rapidly over a wide range of focal positions, which may desensitize the method. Conversely, too low a peak power will result in either no phase change (ie no boiling) at all or one that is extremely sensitive to focal positions, in which case the dynamic range for the method might be impaired, although the method would not be completely useless.
• the optimum pulse energy is found to be in the range of 5 to 50 mJ with a pulse duration in the range of 100 to 500 ⁇ 8. The optimum pulse energy for other types of target will be able to be determined by the skilled man by calculation or trial and error.
• a portion of the pulses are used.
• a portion of the signal eg a certain number n of microseconds worth, at the beginning, end or intermediate part of pulses of Figure 4(a) might be used. might be 10, or any other number.
• a further application of this invention is to use it as a tool to map out the contour profile of a workpiece prior to processing.
• the device is used to actively control the focus lens to workpiece height F by minimising the error signal.
• a feedback loop is included from the processing part of the laser 1 to a means for moving the workpiece 5 in the said direction and this is shown schematically by dashed lines 70 in Figure 1.
• the lens can be moved.
• a feedback loop may also include means for detecting the visible light back-reflected as well as the IR light to improve accuracy and flexibility for material processing applications.
• WO 2009/112815 shows how a CMS can be modified to detect visible, as well as IR back-reflection.
• the M factor also called beam quality factor or beam propagation factor, is a common measure of the beam quality of a laser beam. According to ISO Standard 11146 [4], it is defined as the beam parameter product divided by ⁇ / ⁇ , the latter being the beam parameter product for a diffraction-limited Gaussian beam with the same wavelength. In other words, the half-angle beam divergence is
• a laser beam is often said to be "M 2 time diffraction-limited”.
• a diffraction-limited beam has an M 2 factor of 1, and is a Gaussian beam. Smaller values of M are physically not possible.
• the M2 factor of a laser beam limits the degree to which the beam can be focused for a given beam divergence angle, which is often limited by the numerical aperture of the focusing lens. Together with the optical power, the beam quality factor determines the brightness of a laser beam.
• Embodiments provide simplicity, cost and is integral to the fibre laser itself requiring no additional optics around the beam delivery/workpiece area. Additional components around the delivery area are not required reducing complexity, cost and the possibility of a hostile environment for the optics. Integrating into the fibre laser means that all lasers come with the capability to locate focus irrespective of how the laser beam is being delivered.
• the embodiments above use methods of locating the focus position of a fibre using light that is back reflected off a workpiece and transmitted in the cladding of the delivery fibre. It has been found that for high average power lasers (typically of about 500 W or more, although this is not limiting) the focal position and quality can change with time due to absorption of the laser radiation by delivery optics, resulting in a thermal lens effect in the lenses.
• Figures 8 to 11 show a method of tracking change in focal position to allow a user to quantify the temporal shift in focus and to compensate for it.
• Figure 8 shows an example of a waveform that may be used.
• the waveform comprises pulses of full power 60, typically of pulse duration 1 msec of more. Between these pulses are transmitted very short test pulses 61, typically of 50 or 100 ⁇ 8 as shown.
• the effects of the test pulses on a workpiece are shown on a workpiece which is moved relative to the laser are shown at 62 and the effects on the workpiece of the full power pulse are shown schematically at 63.
• the workpiece is moved relative to the pulsing focus laser to act as a target to measure.
• One particular algorithm described as an example only, is as follows.
• the focus is scanned across the workpiece by either moving the workpiece or the laser beam (or both).
• the waveform shown in Figure 8 is used. This comprises very short eg 50 ⁇ 8 test pulses to measure feedback level, hence quality of focus, and relatively long pulses of, for example, at least msec duration at full power which act to heat the lens or lenses of the optical system up.
• the time period of the test pulse is much shorter than the long higher power pulse. Therefore, effectively thermally the optical system experiences a load very close to if it was on at full power from the time elapsed from start up.
• C The feedback at each test pulse is measured and this effectively measures the position of focus.
• Figure 9 shows a typical back reflected signal with focus position. Reference is made to, for example, Figure 6 for an explanation.
• the test of Figure 8 can be run at anywhere along the curve 64 of Figure 9. If the scan is conducted at point A then a null would be produced if there is no significant problem with the thermal effects of the optic.
• Running at point B gives greater sensitivity since the mean error of the signal is on a slope so that the direction of the focus shift will also be indicated.
• the choice of point at which the scan is done is most ideally at a point of greatest slope, such as at point B, but should not in general be too far from the focus point as this might cause damage to a laser.
• Figure 10 shows schematic test results of a system with good thermal performance of the optical system in which there is no change in error and therefore no change in focus.
• the error signal is generally constant.
• Figure 10 shows a system of poor thermal performance where the focus tends to shift with time and this is indicated by a change in error 66 with time.

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